1. Field of the Invention
The present invention relates to a Czochralski crystal pulling process of producing a large single crystal, and a crystal pulling apparatus for carrying out the same.
2. Description of the Related Art
In a Czochralski crystal pulling process, a single crystal held by a seed chuck is pulled up by a seed wire from a crucible supported on a susceptor and filled with a molten material. Crystal pulling speed can be increased by lowering the temperature of the molten material or lowering the temperature of the crystal being pulled up.
In producing a single crystal by the Czochralski crystal pulling process, the temperature distribution on the surface of the molten material, in general, shows that the crystal bottom stays in the solidifying temperature and the molten material temperature along the radial surface increases gradually from the crystal periphery and then decreases to a temperature slightly above the solidifying point at the radially outermost position where the molten material is in contact with the wall of the crucible. Accordingly, if the temperature of the molten material is held at a comparatively low temperature to increase the crystal growth rate in pulling up a crystal by the Czochralski crystal pulling process, the temperature of the molten material in a region near the the wall of the crucible may reach the solidifying point and hence the single crystal cannot stably be pulled up.
There has been an idea that the crystal growth rate can be increased if the temperature of the crystal being pulled up is lowered by intercepting the radiation heat transfer from the crucible or the molten material to the crystal. Techniques intended to increase the growth rate by intercepting the radiation heat transfer from the crucible and the molten material to the crystal have been proposed. For example, a technique (1) proposed in Japanese Patent Publication (Kokoku) No. Sho 57-40119 (U.S. Pat. No. 4,330,362) and a technique (2) proposed in Japanese Patent Publication (Kokoku) No. Sho 58-1080 (U.S. Pat. No. 4,330,361) cover a crucible and a molten material partially with a metal radiation screen, a technique (3) proposed in Japanese Patent Publication (Kokoku) No. Hei 2-31040 employs a radiation screen of a multilayer insulating screening construction, and a technique (4) proposed in Japanese Patent Laid-open (Kokai) No. Sho 64-72984 (U.S. Pat. No. 4,956,153) employs an annular, heat-resistant, heat-insulating plate.
The techniques (1) to (4) intercept the radiation heat transfer from the crucible and the molten material to the crystal to promote the cooling of the crystal so that the crystal growth rate is increased. In the practical application of these techniques, however, the cooling of the crystal is impeded, so the crystal growth rate is reduced. This problem will be explained hereunder.
In pulling up a crystal without using any radiation screen, a just crystallized portion of the crystal is exposed to radiation heat of about 1300.degree. C. emitted by the wall of the crucible and radiation heat of about 1400.degree. C. emitted by the molten material. As the crystal is pulled up, the crystal radiate heat toward a cylindrical tube of a temperature on the order of 500.degree.. In pulling up a crystal using a radiation screen, the crystal is shielded from the direct radiation heat radiated from the wall of the crucible and the molten material; however, the crystal is surrounded by the radiation screen of a temperature on the order of 1000.degree. C., which is higher than that of the cylindrical tube. Accordingly, the radiation screen impedes the heat radiation of the crystal or, rather keeps the crystal at a high temperature. Since the temperature of a portion of the crystal facing the wall of the crucible or the surface of the molten material, in general, is 1000.degree. C. or higher, and hence the radiation heat shielding effect of the radiation screen on the cooling of the crystal is insignificant. Thus, the effect of the radiation screen on impeding the heat radiation of the crystal is greater than the effect of the same on shielding the crystal from the radiation heat radiated from the wall of the crucible and the molten material and, consequently, the radiation screen increases the temperature of the crystal. Accordingly, the previously proposed techniques (1) to (4) are not very effective on increasing the crystal growth rate because the radiation screen has an effect of keeping the crystal at a high temperature and suppressing the crystal growth rate rather than an effect of increasing the crystal growth rate.
A technique (5) proposed in Japanese Patent Laid-open (Kokai) No. Sho 55-56098 (U.S. Pat. No. 4,378,269) employs an annular radiation screening disk for covering the upper surfaces of the molten material, the crucible and the heater, and a cooling cylinder disposed on the annular radiation screening disk. Since the annular radiation screening disk covers the surface of the molten material entirely, a portion of the crystal in which crystal growth is in process, namely, a portion of the crystal in contact with the molten material, is prevented from cooling. If the temperature of the molten material is lowered to increase crystal growth rate, the molten material solidifies in a region near the wall of the crucible and, consequently, the crystal growth rate is reduced rather than increased.